The present invention relates to a silver halide emulsion exhibiting a relatively high contrast, improved in process stability and forced developability even when coated in a multiplayer structure, and a silver halide photographic material and a silver halide color photographic material by the use thereof.
Recently, the demand for improvements in photographic silver halide emulsions has become pronounced, and further, requirements have also been demanded for higher level photographic performance including higher speed, superior graininess and improved pressure resistance.
Methods for enhancing sensitivity of a silver halide emulsion, specifically for enhancing quantum sensitivity include, for example, the use of tabular silver halide grains, as described in U.S. Pat. Nos. 4,434,226, 4,439,520, 4,414,310, 4,433,048, 4,414,306, and 4,459,353; JP-A Nos. 58-111935, 58-111936, 58-111937, 58-113927 and 59-99433 (hereinafter, the term, JP-A means an examined and published Japanese Patent Application). Further, JP-A No. 63-220238 discloses as a means for achieving enhanced sensitivity, a technique for introducing dislocation lines into the interior of silver halide grains.
On the other hand, the trend for enhancing image quality is remarkable and the desire for enhanced sensitivity at a smaller grain size has increased year by year to achieve enhanced graininess of silver halide grains. The foregoing techniques of tabular grains and dislocation lines are effective means for achieving enhancement of sensitivity of smaller silver halide grains. In cases when dislocation lines are applied to tabular grains of a high aspect ratio to exploit characteristics of tabular grains, however, sufficient sensitization could not necessarily be effected for smaller silver halide grains. Furthermore, deteriorations in photographic performance such as tone characteristic or process stability was caused, resulting in a decrease in contrast and making it insufficient to understand design factors for the grain structure.
JP-A No. 8-62754 disclosed small silver halide grains having dislocation lines and exhibiting enhanced sensitivity and high contrast, of which the equivalent circle diameter was 0.6 xcexcm or less and in which the position for introducing dislocation lines was controlled. However, it was proved in the foregoing embodiments that process stability and forced developability were not sufficient when coated in a multiplayer structure.
Specifically, in the field of color reversal photographic materials, an adjustment of sensitivity by processing, so-called forced development (or extended development) is employed to compensate underexposure, in which development of a silver halide grain emulsion exhibiting superior tone stability and tone balance for developing time, enhanced sensitivity and high contrast has been desired. Further, enhancement of color reproduction in relatively dark image portions (i.e., areas having lower lightness) is desired in the market. For example, JP-A 7-209785 disclosed a technique, in which a layer containing colloidal silver and a DIR coupler was provided between respective light-sensitive layers to control developability, thereby leading to enhanced suitability for forced development. In reversal photographic materials directly forming images through reversal development, however, such a DIR coupler formed a dye in images, making it difficult to use the coupler.
As described above, in silver halide emulsions and silver halide color photographic materials known in the art, enhancements in sensitivity, contrast, process stability and forced developability were not be achieved along with improved color reproduction.
Accordingly, it is an object of the present invention to provide a silver halide emulsion exhibiting a relatively high contrast, improved process stability and forced developability, even when coated in a multiplayer and a silver halide photographic material and a silver halide color photographic material by the use thereof.
The above object of the invention can be accomplished by the following constitution:
1. A silver halide emulsion comprising silver halide grains wherein the silver halide grains meet the following requirement:
(a) at least 50% of total grain projected area is accounted for by tabular grains having an aspect ratio of not less than 5;
(b) a coefficient of variation of total grain size distribution is not more than 25%;
(c) an average iodide content of total grains is not more than 4 mol; and
(d) the tabular grains each have major faces and contain dislocation lines in the peripheral region of the major face, and the tabular grains comprising a high iodide which is internal to and along the dislocation lines and an internal region surrounded by the high iodide phase, the internal region being comprised of substantially homogeneous silver halide phase and having an average iodide content of not more than 1 mol %;
2. A silver halide emulsion comprising silver halide grains wherein the silver halide grains meet the following requirement:
(a) at least 50% of total grain projected area is accounted for by tabular grains satisfying the following equation:
Axe2x89xa74xc3x97{square root over (x)}
xe2x80x83wherein xe2x80x9cAxe2x80x9d represents an aspect ratio and xe2x80x9cxxe2x80x9d represents an average equivalent cube edge length, expressed in xcexcm;
(b) a coefficient of variation of total grain size distribution is not more than 25%;
(c) an average iodide content of total grains is not more than 4 mol; and
(d) the tabular grains each have major faces and contain dislocation lines in the peripheral region of the major face, and the tabular grains comprising a high iodide which is internal to and along the dislocation lines and an internal region surrounded by the high iodide phase, the internal region being comprised of substantially homogeneous silver halide phase and having an average iodide content of not more than 1 mol %;
3. A silver halide photographic material comprising a support having thereon a light sensitive layer containing a silver halide emulsion as described in 1 or 2 above; and
4. A silver halide color reversal photographic material comprising a support having thereon a light sensitive layer containing a silver halide emulsion as described in 1 or 2 above.
As a result of extensive study to analyze factors with respect to the above-described items which were proved to be insufficient in the prior art, it was proved that the position to provide the high iodide phase along with introduction of dislocation lines was an important factor and the internal region having a low iodide content and a homogeneous composition promoted solution physical development. It was discovered that it was significant to enhance development activity of the side-face of the tabular grain, in which development was assumed to be initiated, and it was a point of grain design to control the iodide content of from the side-face to the high iodide phase, that is, to allow iodide to be localized in the iodide phase at a relatively high concentration, reducing the iodide content of the side-face portion.
In addition thereto, it was further discovered in tabular silver halide grains having a relatively high aspect ratio that it is important to control the position of the high iodide phase, formed along with the introduction of dislocation lines. Thus, when the high iodide phase is too external (i.e., when the dislocation lines are too short), iodide contents of the side-face and the shell increase, lowering development activity and resulting in low contrast, specifically when coated in a multiplayer structure. To the contrary, the high iodide phase is too internal (i.e., when the dislocation lines are to long), low contrast results. Accordingly, an appropriate design of the position of the high iodide phase (i.e., lengths of the dislocation lines) is important. However, sufficient effects were not achieved only by such a technique; specifically, contrast, process stability and suitability for extended development used in color reversal photographic materials were not attained to sufficient levels. It was further discovered that an internal region, surrounded by the high iodide phase, being substantially homogeneous and having a relatively low iodide content and the average overall iodide content of total grains being not high were necessary for tabular high aspect ratio grains containing a high iodide localization phase and exhibiting higher contrast and superior forced developability, thereby leading to the present invention. Furthermore, homogeneity of grains is significant to enhance effects of the invention. Thus, enhanced homogeneity of not only grain size but also grain thickness or dislocation line length (i.e., the distance the high iodide phase and the side-face) leads to a marked increase of the effects.